The purpose of the current study was to determine whether maternal prolactin (PRL) had any effects on bone formation in the developing rat pup. Because the most prevalent forms of PRL in rats are unmodified and phosphorylated PRL, both recombinant PRL and a molecular mimic of phosphorylated PRL (PP-PRL) were administered to pregnant animals. Blood samples from the dams showed normal estrogen and progesterone and no effect of extra PRL on parathyroid hormone (PTH), calcium, or alkaline phosphatase (AP). In newborn pups, however, there was a 30% decrease in blood AP in both PRL-treated groups, whereas PTH and calcium levels were not different from controls. When primary rat osteoblasts were exposed to both PRLs, AP activity was reduced, with PP-PRL being the more potent form of the hormone. Histological examination of pup bone formation showed reduced calvarial bone and reduced endochondral ossification in pups exposed to PP-PRL. These results are the first to show a direct inhibitory effect of PRL on osteoblast function.
The secretory activation stage of mammary gland development occurs after parturition and converts inactive lobuloalveoli to active milk secretion. This process is triggered by progestin withdrawal and depends upon augmented prolactin (Prl) signaling. Little is known about the Prl-induced transcriptional changes that occur in the mammary gland to drive this process. To examine changes in the mammary transcriptome responsible for secretory activation, we have used transcript profiling of three mouse models that exhibit failure of secretory activation: knockout of galanin (a regulator of pituitary Prl production and a mammary cell autonomous modulator of Prl action); treatment with S179D Prl (a phosphoprolactin mimic); and knockout of a single Prl receptor allele. A significant reduction in expression was observed in genes belonging to 46 gene ontologies including those representing milk proteins, metabolism, lipid, cholesterol and fatty acid biosynthetic enzymes, immune response, and key transcription factors. A set of 35 genes, commonly regulated in all three models, was identified and their role in lactogenesis was validated by examining their expression in response to Prl stimulation or signal transducer and activator of transcription 5 knockdown in the HC11 mouse mammary cell culture model. The transcript profiles provided by these experiments identify 35 key genes (many for the first time) involved in the secretory activation phase of mammary gland development, show that S179D acts as an antagonist of Prl action, and provide insight into the partial penetrance of failed lactation in Prl receptor heterozygous females.
Soluble formazan assays are widely used for cell number assessment. However, in our hands, we observed frequent occasions in which the actual cell number was at odds with the assay reading. In this study, we have determined that (i) a large proportion of the reading obtained in commonly used culture media can be caused by media component amplification of formazan production in a way that cannot be corrected for by media-only controls; (ii) the albumin present in 10% serum can reduce the assay absorbance by 50% so that an actual doubling of cell number can be obscured; and (iii) this latter effect is dependent on the concentration of fatty acids. To counter these problems, we have developed a protocol that gives consistent readings that are fully representative of cell number while retaining some of the original advantages of soluble formazan assays.
In this study, we further investigated the mechanisms by which pseudophosphorylated prolactin (S179D PRL) inhibits the growth of human prostate cancer cells. When treated with S179D PRL for 3 days, LnCAP cells responded by increasing expression of the vitamin D receptor (VDR) and the cell cycle regulatory molecule, p21, whereas PC3 and DU145 cells did not. After 5 days of treatment, both PC3 and DU145 cells responded. Untreated LnCAP cells express the short 1b form (SF1b) of the human prolactin receptor, but DU145 and PC3 cells express only low amounts of this receptor until elevated by treatment with S179D PRL. DU145 and PC3 cells become sensitive to the negative effects of S179D PRL on cell number after induction of the SF1b. Transfection of either SF1b or SF1a into PC3 or DU145 cells made them sensitive to S179D PRL in the 3-day time frame, a finding that was not duplicated by transfection with the long form of the receptor. Treatment of LnCAP cells with S179D PRL increased long-term activation of extracellular signal-regulated kinase 1/2 (ERK1/2). This did not occur in PC3 and DU145 cells until transfection with SF1a/ SF1b. Blockade of ERK signaling eliminated S179D PRLstimulated expression of the VDR and p21 in LnCAP cells and transfected PC3 and DU145 cells. We conclude that initiation of alternative splicing to produce SF1b, and subsequent altered signaling, contribute to the growth inhibitory mechanisms of S179D PRL. This is the first indication of a role for short prolactin receptors in the regulation of cell proliferation. (Cancer Res 2005; 65(16): 7509-15)
Pituitary cells maintained in monolayer culture for 48 h were used for double isotope labeling to study the release of newly synthesized vs. old PRL. The intracellular pathway taken by newly synthesized PRL was studied by autoradiography. For double labeling the cells were first incubated in a 14C-labeled amino acid (4 h) and then in a 3H-labeled amino acid (1 h), each followed by a 1-h chase. Total PRL (RIA) and radiolabeled PRL (immunoprecipitation) were determined in both cells and media. Thre rate of release of PRL (RIA) was stable throughout the experimental period. In unstimulated cells the ratio of 3H- to 14C-labeled PRL in the medium at the end of a 1-h incubation following the second chase was twice that in the cells at the beginning of this incubation, indicating that newly synthesized [3H]PRL was preferentially released. Stimulation with TRH resulted in increased release of older [14C]PRL. The earliest that radiolabeled (14C or 3H) PRL could be detected in the medium was 15--30 min after exposure of the cells to isotope. For autoradiography, cells were given a 5-min pulse of [3H]leucine, followed by chase periods of 30--180 min. Grain counts indicated that mammotrophs maintained in culture for 48 h transported and packaged PRL with the same time course as those preparations studied previously. Analysis of the distribution of total grains per cell in mammotrophs revealed the presence of several functional subpopulations with different numbers of total grains per cell, indicating that some cells were manufacturing and secreting PRL at a very fast rate. It is concluded that 1) preferential release of newly synthesized PRL is due to preferential discharge of newly synthesized granules, since release occurs at a time when labeled hormone is already in the granules; 2) there is no evidence that the established secretory pathway is bypassed, since transport, concentration, and granule formation occur; and 3) preferential release of newly synthesized PRL is the result of functional heterogeneity within the mammotroph population. Functional heterogeneity is illustrated by major differences in both synthetic rates and TRH responsiveness.
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